ESL systems typically include a plurality of ESLs for each merchandise item in a store. ESLs display the price of corresponding merchandise items on store shelves and are typically attached to a rail along the leading edge of the shelves. A store may contain thousands of ESLs to display the prices of the merchandise items. The ESLs are coupled to a central server where information about the ESLs is typically maintained in an ESL data file which contains ESL identification information and ESL merchandise item information. Price information displayed by the ESLs is obtained from a price look-up (PLU) data file. The central server sends messages, including price change messages, to the ESLs through a communications base station (CBS).
One presently available ESL system uses wireless communications to communicate from the CBS to an ESL. In such an approach, the communication from the CBS to the ESL, the downlink, uses amplitude modulated Manchester coded data. For communication from the ESL to the CBS, the uplink, a continuous carrier wave transmitted by the CBS that is remodulated by the ESL and reflected back to the CBS may be utilized. This technique is known as modulated backscatter or remodulated carrier wave (CW). In such a system, the uplink communication is only used for an ESL to acknowledge receipt and correct execution of the message received by the ESL. The uplink information may be modulated as a single continuous frequency lasting 360 milliseconds (ms), for example. Only a limited number of different modulating frequencies are available for the uplink. Application software infers the status of the ESL from a sequence of interrogation messages to which the ESL either responds with an acknowledgment or does not respond at all, allowing the CBS to determine memory integrity, broken display glass or battery condition, push button depression, or the like.